Strengthened glass can be used in a variety of applications that require higher strength than annealed glass. Examples of strengthened glass include chemically-strengthened and thermally-strengthened glass. Thermally-strengthened glass includes both heat-strengthened glass and fully-tempered glass. Chemically-strengthened glass and thermally-strengthened glass both have strained surface regions under compressive stress and an inner region under tensile stress. Chemically-strengthened glass can be manufactured by submerging untreated glass in a molten potassium salt bath. Typical temperatures are 450° C. to 550° C. and a prototypical salt is KNO3. The sodium ions in the glass surface are exchanged with the potassium ions from the bath. This time dependent ion exchange process results in the formation of compressed surface regions on the glass. Thermally-strengthened glass is typically manufactured by heating annealed glass in a furnace to temperatures over 600° C. followed by rapidly cooling the glass. Such thermal treatment induces residual compressive stress at the surfaces of the glass and tensile stress in the center of the glass.
It is generally accepted in the art that thermally-strengthened glass cannot be cut after strengthening. For example, ASTM C1048-04 section 7.9 states, “Heat-treated flat glass cannot be cut after tempering. Fabrication altering the stress distribution, surface or edge shape, or dimensions must be performed before being heat treated.”
As such, conventional methods for applications that require custom sizes for strengthened glass, especially thermally strengthened glass, typically cut the glass to the desired size prior to the strengthening process. After cutting, the custom sized glass substrates are strengthened. Processes for producing strengthened glass parts and products are mature, widespread, and able to meet the needs of many flat glass processors.
Processing many different sizes of glass sheet is less desirable for certain commercial applications, however, because machinery and applications may need to be customized for different sizes of glass sheet. Processing different sizes of glass sheets also decreases the efficiency and throughput for such commercial processes. Processing multiple sizes of glass sheets or substrates can be especially challenging for processes which include depositing coatings on the glass substrates such as vacuum sputtering, dip coating, or slot die coating. Additionally, processing glass prior to strengthening increases the chances of the glass breaking during earlier processing steps.
Despite such shortcomings, however, the art has not heretofore developed commercially meaningful glass fabrication approaches which would allow preparing and processing thermally strengthened glass in standardized, large-scale format with subsequent cutting to custom sizes for particular applications. In addition to the common industry understanding that cutting processes do not work for thermally strengthened glass, some industrial applications have additional technical challenges which make such an approach less certain. For example, coating glass substrates with thin films prior to strengthening may be undesirable for applications that include films that may be altered by process conditions associated with strengthening (e.g., where films may not withstand the temperatures used for thermal strengthening the glass).
Methods for cutting chemically-strengthened are known but typically have resulted in cut glass substrates with unacceptable edge defects. These edge defects are unacceptable for many applications because they greatly reduce the overall strength of the cut glass and can serve as nucleation points for larger cracks. Mechanical steps for processing the edges have been used (e.g., grinding the edges). These mechanical steps produce particles that are unacceptable for many applications. Notably, for example, the generated particles can cause damage to the surface of the glass and coatings formed on the surface of the glass.
U.S. Patent Publication No. 2011/0304899 to Kwak et al. (“Kwak et al.”) acknowledges that tempered glass cannot be cut and that electrochromic devices cannot withstand the process conditions required to temper the glass. Kwak et al. address this problem by forming the electrochromic device on a piece of annealed glass followed by laminating the annealed glass to a piece of strengthened glass. However, the resultant device does not meet the strength requirements for many applications unless a very low thermal expansion glass such as borosilicate glass is used, and borosilicate glass is very expensive relative to other types of glass such as soda-lime glass.
Similarly, U.S. Patent Publication No. 2012/0182593 to Collins et al. discloses strengthening the glass substrate after cutting by laminating to a strengthened piece of glass. This suffers from the same limitations as Kwak et al.
Many different methods for cutting or scribing glass by using laser energy have been previously reported. A common approach is to apply laser energy to a piece of glass under conditions effective to ablate the glass along a desired cutting line. Ablation occurs when the energy being delivered to the glass is sufficient to vaporize the glass. This method typically produces undesirable cracks and debris, and because of the relatively wide heat affected area, the kerf width is not negligible. These drawbacks have prevented the successful application of this type of method to cutting strengthened glass.
U.S. Pat. No. 5,609,284 to Kondratenko and U.S. Pat. No. 6,787,732 to Xuan report kerf-free methods of cutting glass by thermal stress induced scribing. These methods have fewer drawbacks than the ablative methods but required the propagation of a crack along the cut line as well as a method to initiate the crack propagation. Relying on the propagation of a crack along a defect line is not preferred for cutting strengthened glass and especially for thermally strengthened glass, because cracks may propagate uncontrollably or simply reduce the strength of the cut edge making the cut piece of glass unusable.
U.S. Pat. No. 8,327,666 to Harvey et al. (“Harvey et al.”) discloses using a nanosecond laser to cut chemically-strengthened glass in which the nanosecond laser is focused within the thickness of the glass in order to create a line of local defects which is referred to as the “laser induced damage line”. This damage line allows the chemically strengthened glass to be cleaved by propagating a crack along this line. However, cutting thermally-strengthened glass is different from and significantly more difficult than cutting chemically strengthened glass (e.g., due to different compressive and tensile stress properties) and no details on the laser process conditions, evidence, or examples of cutting thermally-strengthened glass are provided. Harvey et al. discloses an example of cutting chemically strengthened glass with a 50 lam thick compression layer using a 355-nm nanosecond Nd-YAG laser. No enabling description or examples are provided for cutting thermally-strengthened glass or for cutting strengthened glass thicker than 2.0 mm. In fact, the disclosed approach would not be successfully applied to thermally-strengthened glass because the tensile stress in the center region of thermally-strengthened glass is much higher and defects or laser induced damage in this region could cause the glass to explode into small fragments. In addition, the propagation of a crack through thermally-strengthened glass would be much harder to control as the mean free path of crack propagation is much smaller than in chemically strengthened glass and the total stored energy is much greater. Finally, even if the laser induced damage line method occasionally yields successfully cut thermally-strengthened glass, the edges created by the crack propagation would be overpopulated with many micro cracks that would greatly reduce the strength of the cut glass and prevent it from passing standard ASTM strength tests required for building and transportation applications.
Generally, part of the laser energy applied to the glass is converted to heat. The amount of glass subject to the laser heat is typically called the heat affected zone. For laser ablation techniques, using a CO2 laser for example, the heat affected zone is quite large and would not be useful for cutting strengthened glass as the strain created by the heat would cause the glass to break. Other techniques for cutting glass consist of creating a defect line or damage line along which the glass can be cleaved. As noted above, Harvey et al. disclose creating a “laser induced damage line” within the thickness of the glass. In order to do this, the laser is focused in the region of the glass under tensile stress and the energy provided by the laser is converted to thermal energy and locally modifies the glass thus creating a defect. Most of the examples provided by Harvey et al. are for glass compositions having a low coefficient of thermal expansion (CTE). Higher CTE glass would be expected to be much more difficult to cut using this method because the strain induced by the local heating could be enough to initiate the propagation of a crack in the tensile region of the glass, where Harvey et al. claims should be the location of the laser induced damage line. In contrast, the methods disclosed herein do not create a damage line in the center tensile region and generally avoid focusing the laser in the region of high tensile stress. It is desirable to avoid or minimize forming a damage line in the center tensile region of thermally-strengthened glass. The methods disclosed herein can address these problems.
Methods for cutting thermally-strengthened glass wherein at least some of the cut pieces of glass have good edge quality and high strength are desired. Methods for cutting chemically-strengthened glass with improved edge qualities wherein at least some of the cut pieces of glass have good edge quality and high strength are also desired. Methods for cutting composites or devices comprising a strengthened glass substrate (e.g., thermally-strengthened or chemically-strengthened glass substrate) wherein at least some of the cut pieces of composites or devices have good edge quality and high strength are also desired.